In known manner, the bending angle is determined by the vertical position of the edge of the punch with respect to the bearing points in the die. In the case of thin-gauge plates, this position can be precisely determined within a tolerance range of a few hundredths of a millimeter over the entire edge length, since even a deviation of 0.04 mm will result in an error of 1.degree.. This imples a very precise retention of the bottom terminal position of the bending punch and precise compensation for the machine and tool tolerances.
A substantial simplification and improvement were provided by the proposal to establish the position of the punch by a vertical displacement of the die bottom, all of the machine-induced influences on the bending angle thereby being neutralized. The required angle .alpha. may then be determined in uncomplicated and repeatable manner by the vertical displacement of the die bottom. The possibility also exists to bend different angles one after another in one operation by a program-controlled or "NC"-controlled displacement of the die bottom. Here, "NC" means Numerical Control, e.g. digital control of the computer program.
It was observed, however, that despite precise setting and persistance of the depth of penetration, e.g. by precise vertical adjustment of the die bottom, bending angle differences intervene during repetition of the bending operation on different qualitatively equivalent plate units. This derives from the fact that the theoretical sharp edge is never formed in the plate during the bending operation, but that this edge is rounded to a greater or a lesser extent. The radius of curvature also effects the bending angle to a quite substantial degree. On the other hand, the curvature varies in the case of two theoretically equivalent plates as a function of manufacture, structure, etc. In other words, two qualitatively equivalent plate units produced by different makers display bending angles differing from each other after processing on the same apparatus and under constant setting of the latter, since the curvatures engendered during bending differ from each other. This is a great disadvantage which could not be eliminated satisfactorily until the present invention provided the means to overcome this problem.